Potassium sulfate is an important fertilizer ingredient and is obtained from many mineral sources, including langbeinite and sylvite ores containing halite impurities which are found near Carlsbad, New Mexico. A method of recovering potassium sulfate from langbeinite (K.sub.2 SO.sub.4.2MgSO.sub.4 plus @4% NaCl) is described in U.S. Pat. No. 2,684,285 issued to W. B. Dancy.
In the method disclosed in the above patent, finely pulverized langbeinite is reacted with potassium chloride to produce potassium sulfate. The reaction of langbeinite and potassium chloride is conducted in a solution approaching saturation with poatssium chloride to obtain a conversion of about 42% of the potassium salts to potassium sulfate. The remaining 58% of the potassium remains in the liquor phase. Crystals of potassium sulfate are separated from the mother liquor (reaction mixture) and the remaining mother liquor which still contains dissolved salts of potassium is evaporated to just short of the point at which sodium chloride crystals would be formed. Submerged combustion evaporation is used to remove water, and vacuum crystallization is used to keep the temperature of the mother liquor below its atmospheric boiling temperature to enhance mixed crystal formation. The resulting slurry is cooled, the mixed crystals are separated, and then recycled to the reaction mixture.
The above method of recovering additional potassium salts from the mother liquor by means of submerged combustion evaporation is energy-intensive, and with escalating fuel costs the evaporation step has become a major cost burden. In addition, water has become a more precious commodity in the desert location and must be conserved to the extent possible.
Under the method of the subject invention the process has been modified from the method described above to eliminate the submerged combustion evaporation of the mother liquor. This step is replaced by a "salting out" crystallization step by which sodium chloride is added to the mother liquor which contains potassium salts to cause additional potassium salts to crystallize, mainly as potassium chloride, and to settle out of the mother liquor. The new method eliminates the evaporation step just described, and results in a substantial energy saving to reduce production costs. It also eliminates stack emissions into the atmosphere formerly associated with the evaporation step and avoids addition of costly emission control equipment which would be required to comply with new government regulations. In addition to the large initial capital cost of such emission control equipment, there is an ongoing added energy cost burden for their operation.
In the production of potassium sulfate, the use of the subject "salting out" process has been found to eliminate about 90% of the fuel energy requirements formerly required for that operation before the "salting out" process replaced the submerged combustion evaporation step. In addition, a significant cost saving is realized by elimination of ammonia to control pH in the evaporators which tended to form strong acids which required neutralization.
DAS KALI, Part II, edited by Dr. Ernst Fulda and published in Stuttgart, Germany (1928) pages 326-329 describes the processing of complex ores to recover magnesium chloride. Sodium chloride is an impurity in the solid mixed salts obtained. A mother liquor from the process is added to the mixed salts, which contain a large amount of potassium chloride, and smaller amounts of magnesium chloride and sodium chloride. The magnesium chloride salt is more soluble and is dissolved into the mother liquor, leaving only the potassium chloride and sodium chloride in crystalline salt form, and these mixed salts are separated from the mother liquor, and further refined at high temperatures to separate the potassium chloride and sodium chloride by conventional crystallization procedure.
The above described process is really a "leaching out" process which is the opposite of the "salting out" process of the subject invention. The starting material is a mixture of solid salts, including magnesium chloride, potassium chloride and sodium chloride. Magnesium chloride is "leached" out, and the potassium chloride and sodium chloride are subjected to high temperatures to separate them by conventional crystallization procedures which are energy intensive. Control of the "leaching out" process is difficult, and the mineral, glaserite (Na.sub.2 SO.sub.4.3K.sub.2 SO.sub.4) is easily formed if an excess of sodium is present. In addition, this process may not be suitable because of water conservation requirements, and because of the reduced product value caused by the unintentional production of the glaserite impurity.
Applicants' process, by contrast, provides careful control of the amount of sodium chloride added back into the potassium sulfate reactors. This amount of sodium chloride added is intentionally kept just below the sodium chloride saturation level so that no sodium salt crystals are sent back to the potassium sulfate reactors with the potassium chloride crystals. Glaserite can be formed at relatively low concentrations of sodium chloride in the potassium sulfate reactors, and is an undesirable impurity which reduces the product value by depressing K.sub.2 O values below those required to meet standard specifications for fertilizer products.